It seems as though one of the things I need to first determine is what tip speed ratio (tsr) I should incorporate in the turbine design. I thought that maybe this was affected to some degree by the airfoil profile used, but maybe it isn't. Numbers are thrown around all over the place, it seems, and for a three bladed rotor 5-7 is suggested (although that seems a large range to pick from). This article (https://netfiles.uiuc.edu/mragheb/www/NPRE%20498WP%20Wind%20Power%20Systems/Aeorodynamics%20of%20Ro tor%20Blades.pdf) was published 9/8/2007 and has some explanation of the math involved in optimizing tsr. These calculations suggest tsr(opt)=4*pi/n where n is the number of blades. So tsr(opt)=4.19 for n=3. But the following sentence claims that if care is taken in airfoil design, then tsr(opt) may be 30% larger than these values! Where does that come from? He uses an approximation that for an n-bladed rotor s/r=1/2 where s is the "length of the strongly disturbed air-stream upwind and downwind of the rotor".
Anyway, according to that article, for a 3-bladed rotor, a tip speed ratio of 5.45 would be appropriate. But I'm not very confident with that because it seems that the number 7 is thrown around much more! And how does the particular airfoil chosen affect this? Why is it suggested that a higher tip speed ratio is necessarily more efficient? If it is too high then wouldn't the blades be traveling through turbulent air?
Hoping to one day understand. -Carl
Assessment of Optimum Tip Speed Ratio of Wind Turbines: http://www.asr.org.tr/pdf/vol10no1p147.pdf (i had problems downloading that article so i used the google cache of it's html version found here: http://64.233.183.104/search?q=cache:6B54na1rXCYJ:www.asr.org.tr/pdf/vol10no1p147.pdf+optimal+tip+sp eed+ratio&hl=en&ct=clnk&cd=2&client=firefox-a)
In the conclusion, the author states "Having analyzed the findings of this study, it can easily be said that the optimum design speed ratio for every profile is different rather than being 7 as pointed out in related literature." The following profiles were analyzed in that report: NACA 4415, LS-1, Clark Y, Gottingen-398, C-80, M-6, Raf-15 and NACA 2212. Unfortunately, not the S822 and S823. It looks like the idea though, is to optimize the power factor by choosing a tip speed ratio optimized for the following losses: profile losses, end losses, whirlpool losses, and blade number losses. Because of the google translation of the pdf, the graphs do not appear, some text is lost, and the equations are a little difficult to read. The following seems to be the driving equation. Cp = Cpschmitz(λA).ηprofil(λA,ε).ηend(λA,z) where ηprofil(λA,ε) = 1 - (λA/ε) with ε = CL/CD (lift coefficient over drag) ηend(λA,z) = ? (google cuts off text) Cpschmitz(λA) = look at table (google doesn't show diagram, but table is available)
The S823 and S822 are supposed to achieve quite low CD. It might be safe to assume that profile efficiency (ηprofil) approaches 1). How much does end efficiency (ηend) affect the choice of tip speed ratio (λA)? Otherwise, according to the table for Cpschmitz, choosing a larger tip speed ratio would, to a lessening degree (improvement tapers off) offer an improved power factor. Then you need to consider blade wear, vibration, noise, etc. Hmm. I'm beginning to think that picking a number for tsr is to some degree, fairly arbitrary...[ Parent ]
The operation of blade selection goes something like this:
Build turbine base, guess size and number of wire turns for test coil to determine coil size, build stator and test, make blades fit machine.
In the end, TSR, the Tip to relative wind Speed Ratio "Falls Out". Personally, I feel quoting the TSR has about as much to so with design of a set of blades as car color has to do with the seat number selection in an airplane. But, I'm hard-headed. The TSR does give a twist for a blade-set. That twist depends on the carving program chosen, in most cases. But, folks normally carve a twist to fit the 'blank' thickness as opposed to building to the real twist specified by a blade carving program.
The UIUC aerodynamics program has a lot of information and a very large database on airfoils for wind turbines. They also have a computer program 'PROPID' that should help with blade design.
I am of two thoughts on your selection of the Selig 822/823 airfoils. I do believe they should give a better power production for the swept area. They are more difficult to duplicate. The same power may be obtained by using a slightly longer blade-set with an easier to carve profile. Blade-sets are very dependent on alternator power profile. Unless one is building near identical machines, or one is satisfied with mediocre performance, molded blades may be more trouble than help. my opinion only.
You are correct in selecting a 'low speed airfoil'. Airplane airfoil development has almost always been into developing lift at higher speeds, the practical limit of airfoils in wind machines is limited by wind speed and rotational velocity. For an easy to carve airfoil in the speed range you may want to look at airfoils like the USA35, used on the Piper Cub or some of Harry Ribblets low speed/high-lift airfoils.
Good luck,
Ron Adventure is just bad planning." -- Roald Amundsen [ Parent ]
I'm really trying to figure out how we can make many wind turbines as cheaply as possible. My experience with carving the 1.2m wood ones for the 500W Hugh Piggot design made me immediately want to tackle this first. Once I have the fiberglass mold, for which I should only need one wood plug, making the blades from fiberglass will hopefully be quicker, need fewer tools, be easier to train someone to do, have more easily repeatable results, be cheaper, and may also result in lighter, longer lasting blades (I think the wood we have available here is a little softer than would be ideal). If I only need to make one plug from wood, then I can take a lot of care and time in making it. Using templates it shouldn't be too difficult to eventually get a more complicated shape like the S822 and S823 employ.
I am aware of a few of the blade carving programs available. One is an excel spreadsheet that is available from Hugh Piggot's site (i believe). I want to understand some of the assumptions being made in these programs in order to be more confident that they are appropriate to use for the two profile blade I am attempting. Basically, if a number of these are going to come out of a mold, and be installed around the country, I want to have some confidence that they are half-way decent. So, sure, there's a lot for me to learn :)[ Parent ]
If that is your goal, then you are asking the wrong group of people. You should be following the proven business model provided by the Chinese. They pretty much have the market cornered on cheap junk.
You also need to remember that cheap in equals cheap out. If you want a quality product, it isn't going to be cheap.[ Parent ]
In the end, TSR, the Tip to relative wind Speed Ratio "Falls Out".
Expanding on this: TSR and choice of blade profile are details, making a modest change to efficiency.
But for a wind turbine, efficiency is not a major issue. Unlike consumable fuel, which must be paid for, wind is free for the collecting. So turbine efficiency, like horsepower in a Rolls Royce engine, merely has to be "adequate". If it's a little low, make the mill a little bigger.
There is a broad variety of easy to calculate, easy to fabricate, blade designs that will get you to (or above), say, 80% of the Betz limit for the turbine proper. It's impossible for ANY blade design to go above Betz. So a "perfectly efficient" blade would only get you another 25%. You could get more additional power by sticking with the simpler blade profile and increasing the blade length by 12%.
Similarly, extreme precision when constructing the blades is also not an issue. The blades just have to be strong enough to hold together, balanced well enough that you can finish that job with some counterweights, and kinda close to a decent profile. If it's a little off a good profile the main effect will be a minor reduction in efficiency. See above.[ Parent ]
